1. Field of the Invention
The present invention relates to a circuit arrangement for actuating a safety system, particularly an airbag system which comprises an energy source for at least one load circuit having a trigger for the safety system, a respective lead-through capacitor in the lead-in and lead-out of the trigger, and at least two series-connected switches each driven by a mechanical delay sensor.
2. Description of the Prior Art
Such a circuit arrangement, particularly for a motor vehicle air bag system, usually comprises a common energy source that supplies the ignition energy for load circuits having electrically-actuated triggers for the individual airbags. The actuation occurs by way of two switches connected in series with one another, whereby one of the switches is connected between the respective trigger and the energy source and the other switch is connected between the trigger and ground. These two switches are actuated by two delay sensors acting independently of one another, so that a triggering of the air bag system only occurs given response of both delay sensors. What is referred to as the two-air criterion is established by the arrangement preceding and following the respective trigger. The mistriggering, accordingly, is only possible when two errors occur simultaneously. A mistriggering is caused only by two shorts to the supply voltage and to ground or one short and a simultaneous, faulty response of only one delay sensor. Such a circuit arrangement is disclosed, for example, in the German application No. 37 05 867.
The electrically-actuated trigger for each air bag is accommodated in an air bag housing together therewith and along with further mechanical components. An electrical lead-in and lead-out for the trigger, that are connected to the two switches, lead into this air bag housing. Since the lead-in and lead-out must be respectively freely laid over a certain path length in the motor vehicle, high-frequency disturbances can be picked up on these lines. High-briefly appearing voltage and current peaks arising as a result thereof could lead to a mistriggering of the trigger. For this purpose, a lead-through capacitor that has one side connected to the housing ground and, therefore, dissipates high-frequency disturbances to ground, is therefore respectively provided in the lead-in and lead-out upon passage through the air bag housing.
Despite the presence of such lead-through capacitors, measurements with all possible operating and error conditions nonetheless repeatedly yielded current flows via the respective trigger by which it can be stressed. Such triggers, in particular, are electrically-actuated, chemical ignition wafers having a chemical propellant charge in which chemical reactions are initiated by each electrical current flow.
The present invention is based on the perception that such a current flow is caused by switch bounce or chatter of the switch at the supply voltage side for triggering the air bag. Such a switch bounce or chatter, for example, is possible given extremely hard breaking. However, the air bag is not yet triggered as a result thereof since the second switch at the ground side would also have to be closed for that purpose. The switch bounce or chatter, however, leads to a charging of the lead-through capacitors preceding and following the trigger. Since the lead-through capacitors lie on the order of magnitude of a nanofarad, this current is not sufficiently high enough to actuate the trigger.
Since the lead-through capacitors, however, themselves exercise a protective effect, they cannot be eliminated.